1. Technical Field
The present invention relates to a planar optical waveguide circuit type variable attenuator used in an optical communication field or the like.
2. Description of the Related Art
For example, a planar optical waveguide circuit type variable attenuator has been used as an optical variable attenuator for optical communication or the like. The planar optical waveguide circuit type variable attenuator is composed of an optical waveguide layer formed on a substrate made of silicon or the like. The optical waveguide layer has cores and a clad (refer to, for example, a nonpatent document 1).
FIG. 10(a) is a plan view illustrating the configuration of an optical waveguide circuit type variable attenuator using a Mach-Zehnder interferometer circuit 30, and FIG. 10(b) is a sectional view along line VIII—VIII in FIG. 10(a). As shown in FIG. 10(b), an optical waveguide layer 3 formed on a substrate 7 made of silicon or the like, is composed of cores (optical waveguides) 1 and a clad 2 surrounding the cores 1. In the planar optical waveguide circuit type variable attenuator shown in FIG. 10(a), the cores 1 constitute the Mach-Zehnder interferometer circuit 30.
The Mach-Zehnder interferometer circuit 30 has at least one (two in this case) input optical waveguide 1a, 1b, an optical branch portion 21a for causing branching of a light beam inputted from the input light waveguides 1a, 1b, at least one (two in this case) output optical waveguide 1c, 1d, an optical coupling portion 21b provided on the input side of the output light guide waves 1a, 1b, for coupling light beams, and two connection optical guide waves 1e, 1f which connect the optical coupling portion 21b and the optical branch portion 21a together and which are arranged in parallel with and are spaced from each other.
In the Mach-Zehnder interferometer circuit 30 shown in this Figure, the optical branch portion 21a and the optical coupling portion 21b are formed respectively close to two cores 1 arranged in parallel with each other and formed by 2×2 direction coupler.
Further, in the optical circuit device shown in FIG. 10(a), the two connection optical waveguides 1e, 1f of the Mach-Zehnder interferometer circuit 30 are formed respectively therein with phase adjusting means 8a, 8a′ for adjusting phases of light beams transmitted through the optical connection waveguides 1e, 1f. These phase adjusting means 8a, 8a′ are formed of, for example, thin film heaters 9a, 9a′ and are provided on the upper side of the clad 2.
The phase adjusting means 8a, 8a′ and phase part connection optical waveguides 1s, 1t formed underneath a zone where the phase adjusting means 8a, 8a′ are formed, constitute phase shifters. It is noted that reference numeral 23 denotes power supply electrodes for the thin film heaters 9a, 9a′. The phase adjusting means 8a, 8a′ have one and the same configuration, and accordingly, when, for example, the phase adjusting means 8a alone is energized, the following operation is effected.
That is, in the planar waveguide circuit type variable attenuator shown in FIG. 10, when the temperature of the phase part connecting waveguide 1s is locally changed under control by the thin film heater 9a serving as the phase adjusting means 8a, the refractive index of the phase connection optical waveguide 1s on the side where the above-mentioned temperature is changed, is changed and accordingly, the effective refractive index of the core 1 in the part where the refractive index is changed is changed. That is, a thermooptical effect having such an phenomenon that the refractive index of quartz glass or the like is changed depending upon a temperature, is utilized, and with this effect, the phase of the light beam transmitted through the core whose refractive index is changed, is changed. Thus, a phase difference is caused between the light beam transmitted through the phase part connecting optical waveguide 1s and the light beam transmitted through the phase part connecting optical waveguide 1t, that is, the light beams transmitted through the optical connection waveguides 1e, if have a phase difference therebetween.
Namely, due to the themooptical effect caused by a heat generated by the thin film heater 9a serving as the phase adjusting means 8a, the effective optical waveguide length of the thus heated phase part connection optical guide 1s is changed so that the phase of the light beam transmitted through the core having the thus changed effective optical guide wave length is changed, and accordingly, the light transmittance of the Mach-Zehnder interferometer circuit 30 can be changed. Thus, the optical circuit device shown in FIG. 10(a) can be used as an optical waveguide circuit type interferometer capable of changing a light transmittance and an optical branching ratio so as to have a function of an optical variable attenuator. It is noted that the phase adjusting means 8a′ is provided as a back-up one adapted to be used when, for example, the phase adjusting means 8a fails.
In this planar optical guide wave type variable attenuator, since the refractive index of quartz glass from which the cores 1 are formed, has a temperature coefficient dn/dT of 10−5 (1/° C.), if the temperature of the core 1 is increased by 20° C. over a length of, for example 5 mm, the effective optical length of the core 1 varies about 1 μm.
FIG. 11 shows a characteristic curve a which exhibits a relationship between an applied power and an insertion loss in the planar optical waveguide circuit type variable attenuator shown in FIG. 10. In view of this characteristic curve a, it is understood that a light attenuation value of about 10 dB is obtained by an applied power of about 430 mW, and a maximum attenuation value of 22.5 dB is obtained by an applied power of about 520 mW. FIG. 11 also shows a characteristic curve b which exhibits a relationship between an applied power and a difference (PDL: polarization-dependence loss) due to a polarization (TE-polarization and TM-polarization) of the insertion loss in the planar optical waveguide circuit type variable attenuator shown in FIG. 10. In view of the characteristic curve a and the characteristic curve b, it is understood that a difference caused by the polarization of an insertion loss is about −2 dB when the optical attenuation value is about 10 dB.
The optical variable attenuator as stated above is used in a light wavelength multiplex transmission (WDM) system, for example, in a main network of an optical communication system. In the WDM system, although a rare earth added optical fiber amplifier is used for simultaneously amplifying a plurality of light wave beams, since the optical amplifying efficiency has a wavelength characteristic, a difference in light intensity is caused, depending upon a wavelength. Further, since a specified light wave beam is separated or inserted, intermediary of a transmission path, a difference in light intensity is also caused, depending upon a wavelength in this case.
Thus, an optical variable attenuator is used in order to enable differences in light intensity to be precisely and dynamically uniform. Since the difference in light intensity depending upon a wavelength is in a range from 0 to 10 dB, a range of light attenuation value usually required for the optical variable attenuator is from 0 to about 10 dB see e.g., “Development of Variable Attenuator” by Sumimoto and others, Showa Electric Wire Review Vol. 52, No. 1(202).